The present invention relates to clusters of control elements or control clusters for controlling a nuclear reactor of the type comprising a spider having a plurality of arms angularly disposed about a vertical axis and each having at least one vertical sleeve defining a vertical bore and a plurality of control elements, each having a longitudinal axis, an upper plug having a longitudinally outwardly extending shank projecting into a respective one of said vertical bores and comprising shoulder means for longitudinal abutment against the sleeve. It relates also to a method of replacement of a control element in a cluster of the above defined type.
The invention is particularly suitable, although not exclusively, for control clusters insertable into fuel assemblies of cooled and moderated light water reactors, and particularly pressurized water reactors. In these reactors, each control cluster consists of a bundle of rods having a high absorption cross section for neutrons. The bundles are each supported by an upper piece called a "spider". Some of these clusters, for example, regulate the reactivity of the nuclear fuel assemblies all through the nominal operation of the reactor and, in particular, control the thermal power ouput of the reactor. Other clusters contain a consumable neutron absorbing material and are introduced into the core only during the first part of a fuel cycle of operation of the reactor. In other clusters, the rods or control elements simply consists of inert plugs designed to close the guide tubes provided in the assemblies, therefore limiting the flow rate of coolant which flows around the fuel rods. Finally, clusters are typically provided for varying the neutron spectrum in the core by modifying the moderator volume in the core of the reactor.
Control clusters of the above-defined type are already known. FR-A-2 439 457, relating to a cluster and a method of replacement of a control element in a spider, said control element having a longitudinally outwardly extending shank projecting from one end of the cross-head or spider, shows a device wherein the shank of the control element or rod is fitted inside a bore pierced in the spider. A fastening piece retains the shank inside the bore and is fixed by deformation on the spider, this fixation having a pre-determined resistance to downward longitudinal movement. By overcoming the retaining force in applying downwardly a greater force in the longitudinal direction, the rod will be driven out of the bore and freed. More precisely, the fastening of the rod to the spider is achieved by deformation of the end portion of the shank against the surface of the upper portion of the bore, which presents a larger cross-section than the lower portion of said bore.
The end portion of the shank is further deformed to enable it to again pass the narrower lower portion of the bore to release the rod to be replaced.
This type of cluster and the method of replacement of the rod thereof have several drawbacks.
During the positioning or withdrawal of a rod, a thrust force should be applied on the bore formed in the spider. This could result in deformations of the bore itself. It hence could become necessary to repair the spider when the number of rod replacements for a given bore is not limited a priori. These repairs of a radioactivated cluster (the activation is due to neutron bombardment of the spider during its stay in the core of the nuclear reactor) should always be avoided; this is not possible with the prior art devices described hereinabove.
It is also to be feared that the crushing by deformation of the end portion of the shank weakens this end portion because the forces applied during a deformation are not always exactly known; this weakening can even cause an undesirable separation between the rod and the spider during operation of the reactor.
The risk of dropping a rod subjected to multiple vibrations or any other transverse movements bringing the rod into repeated contact with its guiding elements in the assembly, even when the cluster is immobile vertically, and this during the operation of the reactor, therefore appears greater than in the case, for example, of fastening by screwing.
Finally, to drive the rod out of the sleeve during the replacement, a considerable force must be applied to push the shank through the bore. Assuming that the shank is not driven by percussion, which would be damaged for the rod, it is to be feared that jamming of the shank inside the bore could create a considerable shock, risking fissuration or other deterioration of the rod which has been particularly weakened due to its irradiation.